The main aim of fitting a prosthesis is to provide the best possible replacement for the lost limb, as far as possible without functional limitations. If a patient has to be provided with an above-knee prosthesis having a prosthetic knee joint, there are many possible ways of designing such a prosthetic knee joint.
A simple passive, monocentric knee joint is particularly simple in terms of design and production, but it only partially simulates the natural knee joint and, in particular, does not permit a natural gait pattern. A securing against buckling of the prosthetic knee joint can be achieved by a suitably stable set-up, that is to say the relationship of the individual prosthesis components to one another and to the body, if appropriate assisted by a locking mechanism in what is called a lockable knee joint. In such a prosthetic knee joint, the knee is always fully extended to the end of the swing phase, as a result of which the monocentric knee joints, because of the need for a statically secure set-up, have no tendency or only a slight tendency to bend, even at the moment of heel strike. This has the effect that the impact load at heel strike is introduced directly into the thigh stump or into the hip, whereas, from the physiological point of view, the natural knee joint flexes by ca. 25° at heel strike, which results in a considerable damping of the heel strike.
In addition to monocentric prosthetic knee joints, there are polycentric prosthetic knee joints which, with a suitably high position of the instantaneous center of rotation of the extension position, have excellent inherent stability not only during standing, but also when a load is placed on the heel at the start of the stance phase of walking, with the result that the prosthetic knee joint is secure in the extended position, even without a hip extension moment being applied. A polycentric prosthetic knee joint of this type likewise permits a smooth transfer into the swing phase and an initiation of a knee flexion with loading on the front of the foot. An approximation to the natural gait pattern is thereby achieved. Polycentric prosthetic knee joints often have damping devices with which it is possible to initiate an elastically cushioned or damped flexion of the knee without loss of stability. A polycentric prosthetic knee joint with an adjustable pivot stop is described in DE 40 04 988 A1.
Furthermore, so-called active prosthetic knee joints are known which, with the aid of a motor, initiate the flexion and extension on the basis of sensor data. There are also active damping devices present in order to adapt the damping to the particular requirements. Such prosthetic knee joints are extremely complicated in terms of design and production.